How can I calculate how much gaseous hydrogen storage is required at pressure Y psi to conduct a certain number of vehicle fills to 350 bar or 700 bar?

This is not an easy question since many factors influence how much hydrogen can be transferred from one vessel at a higher pressure to another one at a lower pressure and the rate at which it can be transferred. The pressure in the higher vessel will fall while that in the lower vessel will rise as gas is transferred, so the flow rate will typically slow down and eventually stop as the pressures equalize. Fueling protocols have important upper and lower bounds on the flow rate or pressure change rate that is allowed when filling a vehicle. The SAE J2601 family of standards detail those rates and ending pressures.  Most hydrogen dispensers use a cascade method of fueling (switching between multiple supply tanks) to maintain the desired flow rate and maximize the amount of hydrogen that can be supplied. As a general rule, a higher percentage of gas can be transferred from the supply tanks when filling lower pressure 350 bar tanks than higher pressure 700 bar tanks.  Similarly, as storage pressure Y psi is increased, a higher percentage of the gas will be usable for either fill pressure.

The density of hydrogen does not vary linearly with pressure since it’s not an ideal gas. For reference, the density of gas at 350 bar is 24 g/liter and at 700 bar is 40 g/liter. Thus, doubling the pressure only results in a 60% increase in storage volume. For this reason, 700 bar vehicle tanks are rarely filled from a single tank and either use multiple supply tanks in a “Cascade” type system or are direct filled from compression systems.